JP2002088455A - METHOD FOR MANUFACTURING Ni-BASE ALLOY HAVING EXCELLENT HIGH TEMPERATURE SULFIDATION CORROSION RESISTANCE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method, particularly forging and heat treatment methods, by which the high temperature sulfidation corrosion resistance of an Ni-base alloy used for corrosion resisting high temperature equipment members, such as a high temperature sulfidation corrosion resisting Ni-base alloy of examined publication number H9-227975 and Waspaloy (R) ( a registered trademark of the United Technologies), can be improved while maintaining its high temperature strength characteristics at a level equivalent to that of the conventional only. SOLUTION: The method for manufacturing the Ni-base alloy having a composition consisting of, by mass, 0.005-0.1% C, 18-21% Cr, 12-15% CO, 3.5-5.0% Mo, <=3.25% Ti, 1.2-4.0% Al and the balance essentially Ni and having high temperature sulfidation corrosion resistance comprises steps of: finish hot working at a temperature not higher than the carbide solid solution temperature; solution heat treatment at a temperature not higher than the carbide solid solution temperature and also not higher than the recrystallization temperature; and stabilizing treatment and aging treatment at a temperature not higher than the carbide solid solution temperature and also not higher than the recrystallization temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a device used in a corrosive environment at a high temperature, particularly in a sulfide corrosive environment containing H ₂ S and SO ₂ , for example, an exhaust gas from a fluidized bed catalytic cracking device of an oil refinery. The present invention relates to a method for producing a heat-resistant alloy having excellent resistance to high-temperature sulfidation corrosion used in an expander turbine or the like that recovers and uses energy from a fuel.

[0002]

2. Description of the Related Art Conventionally, Ni-based heat-resistant alloys having excellent strength and corrosion resistance at high temperatures have been used for members used at high temperatures such as rotors of expander turbines, and a typical example thereof is Waspaloy (trademark of United Technologies). Known alloys are used.

[0003] These Ni-based heat-resistant alloys used at high temperatures have obtained high-temperature strength by precipitation strengthening of an intermetallic compound called a γ 'phase. Since the γ 'phase has a basic composition of Ni ₃ (Al, Ti), Al and Ti are usually added to these alloys.

On the other hand, in high-temperature equipment exposed to a combustion gas atmosphere such as a turbine or a boiler, sulfate, V
High-temperature corrosion called “hot corrosion” involving a molten salt containing Cl, Cl and the like is known. In addition, it has been reported that sulfidation corrosion due to the direct reaction of gas and metal without the involvement of molten salt occurs at about 700 ° C or higher for Ni-based alloys, which is due to the formation of low melting point Ni-Ni ₃ S ₂ eutectic. It is said to be one of the causes.

[0005] In order to save energy in a petroleum refining plant, a system for recovering energy of exhaust gas discharged from a fluidized bed catalytic cracking apparatus has been developed.
When using Waspaloy, a typical Ni-based super heat-resistant alloy, for the gas expander turbine blades of such a device, the root portion of the rotor blade was used despite its use in a temperature range lower than the temperature at which the conventional problem was encountered. Sulfidation corrosion occurred on the steel.

As a result of observing this phenomenon in detail, it was found that although the corrosion proceeded along the crystal grain boundaries, no molten salt was present at the corrosion site, and the corrosion was caused by a direct reaction between the metal and the gas. Became. In the sulfide gas environment where no molten salt exists in the temperature range below the eutectic melting point of Ni-Ni ₃ S _2, such intergranular sulfide corrosion was hardly observed.

To solve this problem, Japanese Patent Application Laid-Open No. 9-227975
The issue of the inventors, the influence of alloying elements on sulfide behavior of Waspaloy in Ni-Ni ₃ S ₂ sulfide gas environment in the following temperature range eutectic is studied in detail, grain boundaries inside the alloy, including It was revealed that Ti, Al, and Mo contained in the alloy were concentrated in the sulfide layer, and that the content of Ti and Al in the alloy had a significant effect on the high-temperature sulfidation corrosion resistance of the alloy. .

As a result, as disclosed in JP-A-9-227975, Co is 12 to 15%, Cr is 18 to 21% and Mo is 3.5 to 15%.
5%, C 0.02-0.1%, Ti 2.75% or less, Al 1.6%
A high-temperature sulfur-corrosion-resistant Ni-based alloy consisting essentially of Ni, excluding impurities and including the remainder, has been proposed.

[0009]

The alloy disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 9-227975 is an alloy which has been known as an additive element of Waspaloy as an alloy having improved high-temperature sulfidation corrosion resistance of a Ni-base heat-resistant alloy. Among them, especially after examining the ratio of Al and Ti in detail, reducing the Ti content and increasing the Al content has attracted attention as a material that can dramatically improve high-temperature sulfidation corrosion resistance. .

However, even if the alloy disclosed in Japanese Patent Application Laid-Open No. 9-227975 has an improved resistance to high-temperature sulfidation corrosion, if its production method is different, the resistance to sulfidation corrosion, especially It has been clarified by the present inventors that the corrosion resistance at the crystal grain boundaries, that is, the intergranular sulfide corrosion resistance, changes. This finding also applies to previously known Waspaloy.

[0011] The heat treatment conditions for these Ni-base heat-resistant alloys are often determined mainly from the viewpoint of strength characteristics and hot workability, and are not always optimal for high-temperature sulfidation corrosion resistance.

Therefore, an object of the present invention is to provide
Improves high-temperature sulfide corrosion resistance while maintaining high-temperature strength properties of conventional Ni-base alloys used for high-temperature sulfide corrosion-resistant Ni-base alloys disclosed in No. 7975 and materials for corrosion-resistant high-temperature equipment such as Waspaloy It is an object of the present invention to provide a manufacturing method, particularly a finish hot working and heat treatment method.

[0013]

Means for Solving the Problems The present inventors examined the intergranular sulfidation corrosion characteristics of high-temperature sulfidation-corrosion-resistant Ni-based alloy and Waspaloy disclosed in JP-A-9-227975 subjected to various heat treatments. As a result, the grain boundaries are corroded because carbides mainly composed of Cr precipitate at the grain boundaries, so that Cr diffuses from near the grain boundaries and forms a Cr-deficient layer along the grain boundaries. Was found. Therefore, it is determined that if the formation of the Cr-deficient layer at the grain boundaries can be suppressed, the sulfurization corrosion of the grain boundaries can be suppressed.
The present invention has been reached.

That is, according to the present invention, C: 0.005 to 0.5% by mass.
1%, Cr: 18-21%, Co: 12-15%, Mo: 3.5-5.0
%, Ti: 3.25% or less, Al: 1.2 to 4.0%, the balance being a Ni-based alloy substantially consisting of Ni, after finishing hot working at a carbide solid solution temperature or lower. The present invention provides a method for producing a Ni-based alloy having excellent resistance to high-temperature sulfidation corrosion, in which a solution treatment at a temperature equal to or lower than a carbide solid solution temperature and a temperature equal to or lower than a recrystallization temperature is performed, followed by stabilization treatment and aging treatment.

Preferably, the stabilization treatment is 860 ° C. or more and 920 ° C.
1 hour to 16 hours at ℃ or lower, aging treatment is 680 to 760 ℃
The reaction is carried out under the following conditions for 4 to 48 hours, more preferably 620
This is a method for producing Ni-base alloys with excellent high-temperature sulfidation corrosion resistance, in which secondary aging treatment is performed for 8 hours or more at a temperature of -20 ° C or higher to -20 ° C or higher.

A preferable alloy composition of the above-mentioned Ni-based alloy includes, by mass%, Ti: 2.75% or less, and Al: 1.6 to 4.0%, more preferably, B: 0.01% or less, Zr: 0.
This is a method for producing a Ni-based alloy having excellent resistance to high-temperature sulfidation corrosion containing any one or more of 1% or less.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, the present invention
As a result of examining the intergranular sulfidation corrosion characteristics of Ni-base alloy and Waspaloy, which are resistant to high-temperature sulfidation corrosion disclosed in 227975, the grain boundaries are corroded because carbides mainly composed of Cr are precipitated at the grain boundaries. It was found that the diffusion of Cr from the vicinity of the grain boundary caused the formation of a Cr-deficient layer along the grain boundary. Is determined to be able to be suppressed.

The first point of the present invention is that in a Ni-based alloy having a specific composition, the finishing hot working temperature is set to be equal to or lower than the carbide solid solution temperature. Note that the carbide in the present invention refers to a Cr carbide.

As a result, it is possible to obtain a forged structure in which carbides are present during the finishing hot working. In the finishing hot working at a temperature below the carbide solid solution temperature, some of the undissolved Cr carbides that are already present form a solid solution and at the same time a new
Cr carbide precipitates at the grain boundaries. Therefore, the surrounding area is initially Cr
Although a deficiency layer is formed, the diffusion of Cr progresses due to the high temperature maintained during the finishing hot working, so that the Cr deficient layer near the Cr carbide present during the finishing hot working recovers. If cooling after finishing hot working is slow, Cr carbide may precipitate a little at the grain boundaries and a Cr-deficient layer may be formed, but this may occur during the solution treatment after finishing hot working. Can be recovered by diffusion of Cr.

Further, in the finishing hot working at a relatively low temperature lower than the carbide solid solution temperature, distortion due to the finishing hot working remains, and the diffusion of Cr during the subsequent solution treatment and stabilization is promoted, and the Cr-deficient layer is formed. It works favorably for recovery.

In the case of hot working, for example, when hot working is forged, the forging is roughly divided into a steel ingot (an ingot) into a billet (an intermediate shape such as a billet or a bloom); From the piece, it can be further divided into a finish forging closer to the final shape, and the present invention thus defines a finish hot working that approaches the final shape. The hot working in the present invention includes various hot workings such as forging, rolling, drawing, and extrusion. However, the alloy composition specified in the present invention is applied to, for example, a relatively large disk or the like. In many cases, in that case, the material to be hot-worked is also large, and the finish at a relatively low temperature specified in the present invention has a high deformation resistance, so that the finish at a low temperature is required. It is most suitable for forging because the temperature during hot working is easily kept at a low temperature.

Next, the solution treatment is performed on the Ni-base alloy that has been subjected to the finish hot working described above. The second essential point of the present invention is that the solution treatment temperature is set to be lower than the carbide solid solution temperature and lower than the recrystallization temperature. That is, a solution treatment is performed. The purpose of this process is
In addition to the purpose of dissolving the γ 'forming elements Ti and Al, the most important purpose is to leave the Cr carbide obtained by hot working (Cr
To prevent the formation of new grain boundaries due to recrystallization, so that the new Cr in the stabilization and aging treatments after this solution treatment is performed.
Precipitation of carbide can be suppressed to a minimum.

That is, in the subsequent stabilization treatment and aging treatment described below, precipitation of Cr carbide at the crystal grain boundaries is inevitable, but when the Cr carbide forms a solid solution during the solution treatment, the stabilization treatment and aging treatment are carried out. It precipitates again during the aging treatment and forms a Cr-deficient layer. On the other hand, when the Cr carbide obtained by the finishing hot working remains, the amount of precipitation of the Cr carbide on the grain boundaries during the stabilization treatment or the aging treatment decreases, and the Cr-deficient layer also decreases.

Further, even if the solution treatment temperature is lower than the carbide solid solution temperature, if the austenite crystal grains of the matrix are recrystallized to form new grain boundaries, the grain boundaries are formed without carbide precipitation. When a stabilization treatment and an aging treatment are performed after the solution treatment, a large amount of new Cr carbide precipitates at the grain boundaries, and as a result, a large amount of Cr-deficient layer is formed. Since the Cr-depleted layer formed in the steel cannot be sufficiently recovered without a stabilizing treatment and an aging treatment for a considerably long time, the product is poor in sulfurization corrosion. Therefore, the solution treatment is performed at a temperature lower than the recrystallization temperature to prevent the generation of new austenite crystal grain boundaries during the solution treatment.

In addition, the residual strain due to the finishing hot working at a low temperature and the solution treatment at a temperature lower than the recrystallization temperature promotes the diffusion of Cr during the stabilizing process and works advantageously for the recovery of the Cr-depleted layer. .

Next, in the present invention, after the solution treatment described above,
Perform stabilization processing and aging processing. The main purpose of the stabilization treatment and the aging treatment is to improve the strength of the alloy, but if the treatment is performed within the conditions of the stabilization treatment and the aging treatment defined as a preferable range in the present invention, in addition to the strength improvement, It is also possible to improve corrosion resistance. In other words, the stabilization process is performed by setting the temperature and time so that Cr carbide precipitates and Cr diffusion can sufficiently occur, compared to the conditions under which the stabilization process was conventionally performed (for example, 843 ° C x 4 hours, air cooling). A new Cr carbide is sufficiently precipitated therein, and at the same time, Cr can be diffused. Therefore, Cr diffuses into the Cr-deficient layer due to the precipitation of the Cr carbide, and the Cr-deficient layer can be recovered. In this way, the Cr-deficient layer is recovered again during the stabilization process, and by precipitating more Cr carbide at this stage, the precipitation of new Cr carbide during the subsequent aging (hardening) process and This can minimize the formation of a Cr-deficient layer.

However, even if the above stabilization treatment is performed, if the aging (hardening) treatment conditions are not appropriate, precipitation of new Cr carbide and formation of a Cr-depleted layer will occur, and the sulfide corrosion resistance of the alloy will be reduced. Will deteriorate. Therefore,
By setting the age hardening treatment conditions lower than the conventional conditions (for example, 760 ° C. × 16 h, air cooling), it is possible to suppress the precipitation of Cr carbide.

Although the stabilizing treatment and the aging (hardening) treatment conditions greatly affect the strength characteristics of the alloy, the heat treatment conditions of the present invention are set on the assumption that sufficient strength characteristics can be obtained. That is, while the conventional heat treatment conditions were selected with emphasis on strength, the heat treatment conditions described above were obtained as a result of detailed examination as conditions that emphasized the corrosion resistance of the alloy and that sufficient strength could be ensured. is there.

The stabilizing process and the aging process will be described in more detail.

The present inventors have found that the formation of a Cr-deficient layer by precipitation of Cr carbide at the crystal grain boundaries of the alloy is significantly promoted in a temperature range higher than 760 ° C. and lower than 860 ° C., as will be described in Examples described later. These studies have clarified this. Therefore, by performing the stabilization treatment at a temperature higher than this temperature range, the Cr carbide is precipitated as much as possible at the grain boundary and the Cr deficient layer is not formed, and by performing the aging (hardening) treatment at a temperature lower than this temperature range, Cr carbide precipitation at the alloy crystal grain boundary can be suppressed, and the resistance to high-temperature sulfidation corrosion can be improved.

On the other hand, the stabilizing treatment and the aging (hardening) treatment play a role in promoting the precipitation and growth of the γ ′ phase which contributes to the high-temperature strength of the alloy. However, the stabilization temperature
If the temperature is higher than 920 ° C., the γ ′ phase is remarkably coarsened, and the high-temperature strength decreases. Even if the temperature is between 860 ° C and 920 ° C,
If less than the time, the precipitation and growth of the γ 'phase are insufficient,
If it is longer than 16 hours, the γ 'phase becomes coarse and the high-temperature strength decreases. Therefore, the stabilization conditions were set at 860 ° C to 920 ° C for 1 hour to 16 hours.

In the aging (hardening) treatment conditions, in a temperature range lower than 680 ° C., the precipitation and growth of the γ ′ phase are insufficient and the high-temperature strength is insufficient. Even in the temperature range of 680 ° C to 760 ° C, if the time is shorter than 4 hours, precipitation and growth of the γ 'phase are insufficient, and if the time is longer than 48 hours, carbide precipitation at the alloy crystal grain boundary is promoted. You. Therefore, the aging (hardening) treatment conditions were specified at 680 ° C. or more and 760 ° C. or less for 4 to 48 hours.

In the present invention, it is more preferable to perform the secondary aging treatment at a temperature of aging (hardening) treatment minus 20 ° C. or less to 620 ° C. or more for 8 hours or more. In other words, secondary aging
(Curing) treatment is a treatment in a temperature range lower than the aging (curing) treatment temperature. By this secondary aging (hardening) treatment, precipitation strengthening by fine γ 'phase can be further promoted without precipitating Cr carbide at grain boundaries, and strength can be further increased without impairing sulfidation resistance It is.

The temperature of this secondary aging (curing) treatment is 620 ° C.
When the temperature is lower, the precipitation of the γ 'phase hardly occurs and the effect of increasing the strength is not seen, and the temperature of the secondary aging (hardening) treatment is increased.
(Curing) Aging (curing) when the processing temperature exceeds minus 20 ° C
Since the γ 'phase precipitated during the treatment is coarsened and does not contribute to the effect of strengthening the precipitation of the fine γ' phase, the upper limit temperature of the secondary aging (hardening) treatment was set to the aging (hardening) treatment temperature minus 20 ° C.
Also, if the processing time of this secondary aging (curing) treatment is short,
The treatment time of the secondary aging (hardening) treatment was set to 8 hours or more because the effect of the precipitation of the fine γ 'phase contributing to the precipitation strengthening was reduced.

As described in detail above, by using the manufacturing method of the present invention, it is possible to improve the high-temperature sulfidation corrosion resistance and to impart excellent strength at high temperatures, but the characteristics are sufficiently exhibited. In order to achieve this, it is also important to simultaneously optimize the alloy composition necessary for improving the high-temperature sulfidation corrosion resistance of the alloy itself.

The alloy composition suitable for use in the present invention will be described below. In this specification, mass% is used unless otherwise specified.

C forms Ti and TiC, and Cr and Mo form M ₆ C
, Forming a M ₇ C ₃ and M ₂₃ C ₆ type carbides, these carbides suppressing the coarsening of grain size. Furthermore, M ₆ C and M ₂₃ C
₆ is an essential element in the present invention in order to strengthen the grain boundary by precipitating an appropriate amount at the grain boundary. But C is 0.005
If not more than 0.1%, the above effect cannot be obtained. If it exceeds 0.1%, not only the amount of Ti required for precipitation strengthening decreases, but also the amount of Cr carbide precipitated at the grain boundary during the stabilization treatment becomes too large. The boundaries become weak, and it takes a long time to precipitate Cr carbide at the grain boundaries and recover the Cr-depleted layer. Therefore, C is 0.005 to 0.1%
Limited to.

Cr forms a stable and dense oxide film in a corrosive environment in which an oxidizing action such as oxidizing acid and high-temperature oxidation works simultaneously in the atmosphere, and improves the oxidation resistance. In addition, it has an effect of increasing the high-temperature strength by precipitating carbides such as Cr ₇ C ₃ and Cr ₂₃ C _{6 in combination with} C. However, if the Cr content is less than 18%, among the above effects, particularly the oxidation resistance is insufficient, and if the Cr content exceeds 21%, the formation of harmful intermetallic compounds such as the σ phase is promoted. Therefore, Cr was limited to 18-21%.

Co acts itself as a solid solution to strengthen the matrix (base) as a solid solution in the Ni-based alloy, but further reduces the solid solution amount of the γ ′ phase in the Ni-based matrix to reduce the amount of γ ′ precipitated. The effect of the strengthening action is exhibited by increasing. However, if the content of Co is less than 12%, the above effect is insufficient. If the content of Co exceeds 15%, harmful intermetallic compounds such as σ phase are generated, and the creep strength is reduced.
Therefore, Co was limited to 12-15%.

Mo mainly forms a solid solution in the γ phase and the γ ′ phase to increase the high temperature strength. It also improves the corrosion resistance to hydrochloric acid and the like. However, if the content of Mo is less than 3.5%, the above effect is insufficient. If the content of Mo exceeds 5.0%, the matrix structure is destabilized. Therefore, Mo was limited to 3.5% to 5.0%.

Ti and Al are mainly converted into Ni ₃ (Al, Ti) to form γ ′
It is an important element that forms a phase and provides precipitation strengthening. However, the higher the amount of Ti, the more sulfur corrosion inside the alloy is promoted. Therefore, the upper limit of Ti was set to 3.25%. The more preferable upper limit of Ti that can suppress the promotion of sulfide corrosion is 2.75%. On the other hand, if the Ti content is too small, it becomes difficult to maintain the required high-temperature strength, so it is preferable to contain 0.5% or more.

When Ti is contained in the above range, it is necessary to add 1.2% or more of Al in order to form a sufficient amount of γ 'phase and maintain high-temperature strength. An increase in the amount of Al is effective not only in high-temperature strength but also in improving sulfidation resistance. However, excessive addition of Al causes elongation at high temperatures, reduction in drawing, and reduction in hot workability. Therefore, the upper limit of Al is set to 4.0%. From the balance of high-temperature strength, sulfidation resistance, high-temperature ductility, and hot workability, the lower limit of the Al content is preferably 1.6%. By controlling the contents of Ti and Al in this manner, high-temperature strength and high-temperature sulfide corrosion resistance are improved.

Although not an essential element in the present invention, B or B contains 0.01% or less and Zr or 0.1% or less as an element capable of increasing grain boundary strength and suppressing grain boundary destruction. can do. However, if B and Zr are added in amounts exceeding 0.01% and 0.1%, respectively, the melting point of the grain boundaries is lowered and melting damage is likely to occur, so that the contents are limited to 0.01% or less and 0.1% or less, respectively.

Further, in the present invention, since the finishing hot working temperature needs to be slightly lowered as described above, Mg may be added at a maximum of 0.02% as an element for improving hot workability. However, when added in excess of 0.02%, the melting point is low.
The upper limit is preferably set to 0.02% because Mg intermetallic compounds are easily formed at grain boundaries and hinder hot workability. As an element having the same effect, Ca can also be added in an amount of 0.02% or less.

The following elements may be included in the alloy of the present invention within the ranges shown. P ≤ 0.04%, S ≤ 0.01%, Cu ≤
0.30%, V ≤ 0.5%, Y ≤ 0.3%, rare earth element ≤ 0.02%, W
≦ 0.5%, Nb ≦ 0.5%, Ta ≦ 0.5%

[0046]

The present invention will be described below in more detail by way of examples.

After being melted in an induction heating furnace in an inert atmosphere and cast in an inert atmosphere, 60 × 130 × 10
Φ500mm or φ1400mm simulating a 00mm prismatic forged disk and a gas expander turbine disk
The disk-shaped forging was used as a test material. The chemical composition is shown in Table 1. Alloy A is an alloy disclosed in JP-A-9-227975, and alloy B is an alloy conventionally known as Waspaloy.

[0048]

[Table 1]

These alloys A and B were subjected to forging and heat treatment shown in Table 2, and then evaluated for strength characteristics and resistance to high-temperature sulfide corrosion. What is shown in the “alloy” column in Table 2 corresponds to the alloy in Table 1. In the column of `` forging conditions '', the symbol L is obtained by dividing the steel ingot, repeating forging and finish forging (finish hot working) at 1010 ° C, while the symbol H indicates It is made by ingoting a steel ingot, repeatedly forging, and finish forging (finish hot working) at 1080 ° C.

[0050]

[Table 2]

The relationship between the forging temperature and the carbide solid solution temperature was confirmed. For confirmation, from the sample after forging (forging condition L),
After cutting out a block of 20 mm, the block was heated at a temperature of 1010 ° C. or 1080 ° C. for 4 hours, and the microstructure of the air-cooled block was examined with a scanning electron microscope. The reason for using a small sample here is to increase the cooling rate and avoid precipitation of new Cr carbide during cooling. The result of examining the microstructure with an electron microscope is shown in FIG. After heating at 1010 ° C, carbides are present at the grain boundaries (Fig. 1 (a)).
It can be seen that the solid solution was almost completely formed by heating at a temperature of ° C. (FIG. 1B). Therefore, in this case, forging condition L corresponds to the condition of forging at a temperature equal to or lower than the solid solution temperature of carbide.

Next, the relationship between the solution treatment temperature and the recrystallization temperature was examined. The sample after the forging (forging condition L) was heated at a temperature of 1010 ° C. or 1040 ° C. for 4 hours, and the microstructure was examined in the same manner as above. The result is shown in FIG. Heating temperature is 1010
At ℃, recrystallization hardly occurred (FIG. 2 (a)),
Since recrystallization almost occurred at 1040 ° C. (FIG. 2B), it was confirmed that the recrystallization temperature was in a temperature range of more than 1010 ° C. and less than 1040 ° C.

Then, finish-forged alloys A and B
From the test material, blocks were cut out in a size sufficient to collect various test pieces, and subjected to various heat treatments shown in Table 2, and then various test pieces were prepared, and their strength characteristics and high-temperature sulfurization corrosion resistance were evaluated.

The strength properties were evaluated based on the tensile properties at room temperature and 538 ° C. and the creep rupture properties at a temperature of 732 ° C. and a stress of 517 MPa. The test specimens have a high temperature sulfidation corrosion resistance of 60
N ₂ -3% H ₂ -0.1% H ₂ S at 0 ℃ 588M in mixed gas atmosphere
Exposure was performed for 96 hours while applying a tensile stress of Pa, and evaluation was made based on the presence or absence of fracture and the depth of intergranular sulfide corrosion generated by observing the cross section. Table 3 shows the strength characteristics and the high-temperature sulfidation corrosion resistance of each test piece.

[0055]

[Table 3]

From the results in Table 3, regarding the mechanical properties,
The value of the conventional alloy (Waspalloy) is at the level of No. 4 and No. 21, and the mechanical properties of the treated alloy of the present invention are almost the same as those of the alloy and sufficient strength is obtained. In addition, the alloys A and B subjected to forging and heat treatment (conditions Nos. 1 to 9) of the present invention have a maximum pit depth of 30 in a sulfide corrosion environment.
Very small, less than μm. On the other hand, forging of the comparative example,
Alloys A and B that have been heat-treated (conditions Nos. 20 and 21) have deep grain boundary erosion of 200 μm or more inside the alloy under stress load, or cannot withstand a 96-hour exposure test and break. Has been done.

Observation of the cross section of the fractured alloy shows that, as shown in FIG. 3, severe intergranular sulfide corrosion is involved, and it is evident that the fracture of the alloy is caused by intergranular sulfide corrosion.

As described above, since the forging heating temperature is low but the solution treatment temperature is high, the solid solution and recrystallization of carbides proceed, and the stabilization treatment following the crystal grain boundaries newly formed by recrystallization and It is considered that carbides were precipitated by the aging treatment and a Cr-deficient layer was formed around the carbides, thereby deteriorating the sulfuration resistance. In Comparative Examples Nos. 22 and 23, the solution treatment temperature was low, but the forging heating temperature was high, so that the high-temperature sulfidation corrosion resistance was not sufficient.

Further, in the present invention, the high temperature sulfidation corrosion resistance of Nos. 5 to 9 in which the stabilizing treatment was performed at 860 ° C. to 920 ° C. and the aging treatment was performed at 680 ° C. to 760 ° C. The depth is 10 μm or less, and the high-temperature sulfidation corrosion resistance is further improved as compared with Nos. 1 to 4.

The reason can be understood from the intergranular corrosion map obtained by the following striker test. This striker test evaluates the degree of Cr deficiency layer formation (intergranular corrosion susceptibility) due to the precipitation of intergranular carbides. Cr
It is considered that the degree of the Cr-depleted layer evaluated by the striker test is proportional to the high-temperature sulfidation corrosion because the Cr-depleted layer is formed near the grain boundary due to carbide precipitation. This was confirmed by comparing the results of the striker test and the high-temperature sulfidation corrosion test.

Table 4 shows the heat treatment conditions of the test pieces subjected to the striker test. As the test piece, a test material of forging condition L of alloy A was used. FIG. 4 shows the weight loss of corrosion in the striker test as a function of temperature and time.
4 shows a grain boundary corrosion region map showing a region where a Cr deficiency layer is formed.

[0062]

[Table 4]

From FIG. 4, the conventional stabilization process of 843 ° C. × 4 h air cooling and the aging process of 760 ° C. × 16 h air cooling are as follows.
It is one of the heat treatment conditions that gives the highest intergranular corrosion susceptibility, and it can be seen that it is not the optimum condition for the high-temperature sulfidation corrosion resistance. On the other hand, it is understood that when the stabilization in a higher temperature range and the aging treatment in a lower temperature range are performed, the intergranular corrosion susceptibility is low, and the high temperature sulfurization corrosion resistance is improved. As described above, the stabilization treatment after the solution treatment is performed at a higher temperature than the conventional condition, and the aging treatment is performed at a lower temperature than the conventional condition, whereby the high-temperature sulfidation corrosion resistance can be significantly improved. This is consistent with the results of Nos. 5 to 9 of the present invention.

From the above results, by subjecting the Ni-based heat-resistant alloy to the forging and heat treatment of the present invention, it is possible to remarkably improve the high-temperature sulfide corrosion resistance while maintaining the same high-temperature strength characteristics as the conventional one. .

[0065]

As described above, the present invention has a higher resistance to high-temperature sulfidation corrosion, particularly to intergranular corrosion, while maintaining sufficient high-temperature strength characteristics, as compared with the conventional production method only considering strength. It is intended to provide a Ni-based alloy having improved susceptibility, whereby a highly reliable device member can be provided in a high temperature sulfide corrosive environment.

In the future, the use environment of high-temperature equipment such as turbines and boilers tends to be severer due to a decrease in the quality of fossil fuel due to a reduction in environmental load and energy saving, and an increase in the efficiency of energy devices. Therefore, the invention relating to the improvement of the corrosion resistance of the device member as in the present case can be said to have an important meaning in the future.

[Brief description of the drawings]

FIG. 1 is an electron micrograph of a grain boundary after heating to each temperature.

FIG. 2 is a micrograph after heating to each temperature.

FIG. 3 is an electron micrograph of a fracture surface after sulfidation corrosion under stress loading conditions.

FIG. 4 is a temperature-time-intergranular corrosion susceptibility curve obtained by a striker test.

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[Procedure amendment]

[Submission date] June 12, 2001 (2001.6.1)
2)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0021

[Correction method] Change

[Correction contents]

In the case of hot working, for example, forging in hot working, the forging is roughly divided into an ingot- forming step of turning a steel ingot (ingot) into a steel slab (intermediate shape such as billet or bloom); The steel forging can be further divided into a finish forging having a shape close to the final shape, and the present invention thus defines a finish hot working that approximates the final shape. The hot working in the present invention includes various hot workings such as forging, rolling, drawing, and extrusion. However, the alloy composition specified in the present invention is applied to, for example, a relatively large disk or the like. In many cases, in that case, the material to be hot-worked is also large, and the finish at a relatively low temperature specified in the present invention has a high deformation resistance, so that the finish at a low temperature is required. It is most suitable for forging because the temperature during hot working is easily kept at a low temperature.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) C22F 1/00 682 C22F 1/00 682 683 683 691 691B 691C 694 694B (72) Inventor Takehiro Ono Yasugi-cho, Yasugi-shi, Shimane 2107-2 Hitachi Metals Co., Ltd. Yasugi Plant (72) Inventor Toshihiro Uehara 2107-2 Yasugi-cho, Yasugi City, Shimane Prefecture Hitachi Metals Co., Ltd. No. 1 Inside Ebara Research Institute, Inc. (72) Inventor Shobo Miyasaka 4-2-1 Motofujisawa, Fujisawa-shi, Kanagawa Prefecture Inside Ebara Research Institute, Ltd. (72) Inventor Shuhei Nakahama Haneda, Ota-ku, Tokyo 11-1 Asahicho, Ebara Corporation (72) Inventor Shigeru Sawada 11-1, Haneda Asahimachi, Ota-ku, Tokyo F-term in Ebara Corporation 3G002 EA06

Claims (5)

[Claims] (1) C: 0.005 to 0.1%, Cr: 18% by mass
~ 21%, Co: 12 ~ 15%, Mo: 3.5 ~ 5.0%, Ti: 3.25%
The following is a method for producing a Ni-based alloy containing Al: 1.2 to 4.0%, with the balance substantially consisting of Ni, after finishing hot working at a carbide solid solution temperature or lower, and then at a carbide solid solution temperature or lower. A method for producing a Ni-base alloy having excellent resistance to high-temperature sulfidation corrosion, wherein a stabilization treatment and an aging treatment are performed after a solution treatment at a temperature below the recrystallization temperature. 2. The stabilization treatment is performed at 860 ° C. or more and 920 ° C. or less.
Time to 16 hours, aging at 680 ° C to 760 ° C
The method for producing a Ni-based alloy excellent in high-temperature sulfidation corrosion resistance according to claim 1, wherein the method is performed under a condition of 48 hours. 3. An aging treatment temperature minus 620 ° C. or more.
3. The method for producing a Ni-based alloy excellent in high-temperature sulfidation corrosion resistance according to claim 1 or 2, wherein a secondary aging treatment is performed at a temperature of 20 ° C for 8 hours or more. 4. In mass%, Ti: 2.75% or less, Al: 1.6 to
The method for producing a Ni-based alloy having excellent resistance to high-temperature sulfidation corrosion according to any one of claims 1 to 3, characterized by containing 4.0%. 5. B: 0.01% or less, Zr: 0.1% by mass%
Claim 1 characterized by including one or more of the following:
5. The method for producing a Ni-based alloy having excellent resistance to high-temperature sulfidation corrosion according to any one of claims 1 to 4.